Programmable logic devices (PLDs) are integrated circuits which increasingly are being used to provide the logic for electronic systems. For example, these devices may be used as "glue" to electrically connect and control the interaction of the major parts of a microcomputer system. Typically, PLDs include a set of input pins, two arrays of logic gates, i.e. an AND array followed by an OR array, and a set of output pins. Frequently, flip-flops following the OR array together with feedback lines are also included in order to provide registered output and sequential logic capabilities instead of the combinatorial logic provided by the AND/OR arrays alone.
Presently, several basic types of PLD architectures are available. In programmable logic elements (PLEs), the AND array is fixed and the OR array is programmable. PLEs are useful in applications requiring most or all possible input combinations, such as lookup tables and character generators. However, because the array size must be doubled for each additional input, PLEs are limited by cost and performance constraints to a small number of inputs. Programmable logic arrays (PLAs) have both a programmable AND array and a programmable OR array. Programmable array logic (PAL) devices have a programmable AND array, but a fixed OR array. Both the PLA and PAL architectures have advantages. Because both arrays are programmable, PLAs offer a high degree of functional flexibility. However, PALs are faster, because a programmable OR array is slower than dedicated OR gates. The PLA's flexibility is useful for complex state.-machine and sequence applications, while most other applications not requiring a high degree of flexibility take advantage of the PAL's speed.
Some attempts have been made to combine both functional flexibility and speed in a PLD architecture. In Monolithic Memories' series of MEGAPALS, the size of the AND array was increased and a fixed number of AND product terms were allowed to be shared amongst two outputs. Altera's EP1200 chip is segmented into "sub-PALs" with only four outputs, the outputs of a particular segment being usable as inputs for only some of the sub-PALs. In each case, all of the inputs are available to all of the AND terms simultaneously, resulting in AND arrays with 64 inputs, most of which remaining unused for any given product term. Because of their fixed product terms, there are 16 product terms per OR gate. In practice, few sets of logic need so many inputs to an OR gate.
In U.S. Pat. No. 4,207,556, Sugiyama et al. discloses a programmable logic array arrangement having a plurality of cell units, each comprising a plurality of electronic elements, such as resistors, diodes and transistors, a wiring matrix of row and column lines, and an array unit having a group of switching elements for selectively interconnecting the various row and column lines, and electronic elements. The arrangement sacrifices density and speed for functionality by including a large number of electronic elements with variable wiring in each unit.
In Ikawa et al., "A One Day Chip: An Innovative IC Construction Approach . . . ", IEEE Journal of Solid-State Circuits, vol. Sc-21, No. 2, April 1986, pp. 223-227, a VLSI chip contains 50-200 standard logic functional blocks of SSI/MSI level integration performing various kinds of functions, such as inverters, NORs, NANDs, flip-flops, shift registers, counters, multipliers, ALUs, etc. Each of these fixed functional units may be connected to other functional units by means of an EEPROM switch matrix. The switch matrix provides flexibility and can easily be reprogrammed, but a large number of standard functional blocks must be anticipated to provide true flexibility, most of which would be unused for any given chip function.
An object of the present invention is to provide a programmable logic device architecture which makes good use of chip area, and combines functional flexibility with speed.